In broadcast radio systems using a digital display, the system uses some type of timing reference, usually in the form of an external crystal oscillator or reference clock attached to the radio or somewhere else in the system. For example, FIG. 1 illustrates a prior art radio receiver system 100 including a fully integrated complementary metal oxide semiconductor (CMOS) single die integrated receiver circuit 102 that is coupled to receive analog radio frequency (RF) signals from an external antenna 104, and to provide analog audio signals to a speaker device.
As shown in FIG. 1, incoming analog RF signals are provided to low noise amplifier (LNA) 106, which in turn provides amplified RF signals to mixer circuitry 108 where a selected frequency of the incoming RF signals is downconverted to a lower frequency (e.g. a low intermediate frequency) by mixing with a signal output from local oscillator in the form of voltage controlled oscillator (VCO) circuitry 122. The frequency of VCO circuitry 122, is controlled by an automatic frequency control circuit (AFC) 127 which receives an incoming clock signal from an external (i.e., off-chip) crystal oscillator 128 and on-chip crystal oscillator core 126 as shown. The downconverted signal is then provided to analog-to-digital conversion circuitry (ADC) 112 through low-pass filter circuitry 110 to remove upconverted high frequency energy produced in mixer circuitry 108. ADC 112 in turn provides a digitized signal to digital signal processor (DSP) circuitry 114, which performs signal processing and/or demodulation operations to obtain the message content in the incoming signal. The digitized message information is then provided to digital-to-analog conversion (DAC) circuitry 116, which provides analog output audio signals to external speakers.
As further shown in FIG. 1, an on-chip microcontroller (MCU) 120 is coupled to control fine tuning of VCO 122 based on digital RF tuning control signals that may be received directly from an external MCU 118 that responds to manual tuning input from a user. On-chip MCU 120 controls the fine tuning of VCO 122 based on the digital tuning control signals to enable the receiver circuit 102 to tune to the desired RF channel. As shown in FIG. 1, frequency counter 124 uses the incoming clock signal from external crystal oscillator 128 and on-chip crystal oscillator core 126 to measure the tuned frequency of VCO 122, and outputs this measured frequency information to an on-chip display controller 130. On-chip display controller 130 in turn provides frequency display signals to off-chip digital display 132, which displays the identity of the tuned frequency to the user.
One variation of generating a local oscillator signal is to wrap a phase-locked loop (PLL) around a VCO for frequency control. PLL synthesizers require crystal oscillators to keep the timing in the system.